Organic semiconductors have attracted a great deal of interest due to their wide range of applications in opto-electrical and switching devices and their ease of processibility as compared to inorganic semiconductors. One class of opto-electrical devices is that using an organic material for light emission (an organic light emitting device or “OLED”) or as the active component of a photocell or photodetector (a “photovoltaic” device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer.
In an organic electroluminescent device, electrons and holes are injected into a layer of electroluminescent semiconducting material where they combine to generate excitons that undergo radiative decay. Holes are injected from the anode into the highest occupied molecular orbital (HOMO) of the electroluminescent material; electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the electroluminescent material. An organic hole injecting material is commonly provided to assist injection of charge from the anode into the electroluminescent layer. In WO 90/13148 the organic light-emissive material is a conjugated polymer, namely poly(p-phenylenevinylene) (“PPV”). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). Light emitting polymers such as polyfluorenes and polyphenylenes are advantageous in that they are solution processable. In particular, solution processable light emitting polymers may be inkjet printed as described in EP 0880303 to produce high information content displays, in particular full colour displays.
The preferred approach in forming full colour OLEDs requires red, green and blue electroluminescent materials. However, a drawback of many blue organic electroluminescent materials is that their emission is a relatively pale blue, due to an insufficiently large HOMO-LUMO bandgap, when compared to the standard blue as defined by 1931 CIE co-ordinates.
Hole transporting co-polymers containing triarylamine “TA” and bis(triarylamine) units “BTA” are disclosed in, for example, WO 99/48160 and WO 03/000773. Examples of these units are shown below:

These materials may be used as hole transporting materials and/or as blue electroluminescent materials. However, the above identified co-polymers have drawbacks. In particular, the HOMO levels of these materials are not ideally matched to the workfunction of the typical anode and/or organic hole injecting material, which negatively affects their hole transporting properties. Furthermore, the blue emission from these materials is a relatively pale blue.
There therefore exists a need for polymers having good hole transporting properties and a deeper blue colour.
EP 1304750 discloses a polymer of formula (A):

No significance is attached to the presence of the methyl substituents.
Likewise, Polymer Preprints (American Chemical Society, Division of Polymer Chemistry) (1997), 38(1), 396-397 discloses a hole transporting polymer of formula (B):

However, the present inventors have found that repeat units such as A and B which have a biphenylene unit between the two nitrogen atoms are not as effective as hole transporters when compared to such a unit with a phenylene unit between the two nitrogen atoms.
U.S. Pat. No. 6,309,763 discloses polymers containing BTA units which can be substituted on the pendant phenyl ring, but are unsubstituted on the phenyl rings which are incorporated in the main chain of the polymer.
WO 00/55927 discloses polymers which can also contain BTA units. However, the substitution of these units is only disclosed on the pendant aryl ring.
WO 99/32537 discloses substituted and unsubstituted triarylamines. However, these are defined low molecular weight compounds for use in organic electroluminescent devices based on small molecules. Corresponding polymers are not disclosed.
WO 99/32537 discloses polymers based on substituted and unsubstituted triarylamines. However, bis(triarylamine) units (BTA) are not disclosed in this application.